1. Field of the Invention
This invention relates to a solid-state image pick-up device which may be employed for a video camera unit or an image scanner for imaging an object using a solid-state image pickup device (CCD image sensor). Note particularly, it relates to a solid-state image pickup device in which the image pickup light radiated on the CCD image sensor is split into an ordinary light and an extraordinary light having its optical axis shifted by a predetermined amount with respect to the optical axis of the ordinary light, and in which imaging may be carried out by shifting the relative position between the CCD image sensor and the image pickup light by electrical means.
2. Description of the Related Art
A solid-state image pickup device, or a CCD image sensor, is extensively used in video cameras or image scanners because of its small size and low power consumption. The CCD image sensor, having a number of pixels on the order of 400,000, is usually mounted on the video camera. However, the CCD image sensor, having the number of the pixels on the order of 400,000, is inferior in resolution to an image tube. Recently, a high resolution television receiver, known as a high-definition (HD) television receiver, has become popular, and a CD image sensor having the number of the pixels increased to 2,000,000 has been developed by reducing the pixel area in order to cope with the HD television receiver. However, if the pixel area is reduced, output image pickup signals of the solid-state image pickup device is lowered in signal level to deteriorate the S/N ratio. In view of the lowering of the S/N ratio, the number of 2,000,00 pixels of the solid-state image pickup device represents a possible upper limit such that it has become difficult to increase the number of pixels to improve the resolution further. On the other hand, since the CCD image sensor having 2,000,000 pixels is insufficient in resolution if it is to be used for inputting a printed text, a demand for a higher resolution has been raised.
For coping with the above problem, a solid-state image pickup device has been developed, in which the relative position between the solid-state image-pickup device (CCD image sensor) and the image pickup light is shifted in an amount equal to one or more integer fractions of the pixel pitch during imaging for increasing resolution by increasing the spatial sampling area without increasing the number of pixels.
With such solid-state image pickup device, the CCD image sensor is mounted on a piezoelectric element and oscillated in one direction from field to field at an amplitude equal to a half pixel pitch for shifting the image pickup light radiated on the CCD image sensor by a pitch equal to a half pixel by way of image shifting for increasing the spatial sampling area for achieving a picture of high resolution without increasing the number of pixels.
There has also been known a solid-state image pickup device in which a thin glass plate is mounted on the front face of the CD image sensor and oscillated at an extreme small angle during imaging. With such solid-state image pickup device, the image pickup light radiated on the CCD image sensor is image-shifted each time the glass plate is oscillated for increasing the spatial sampling area for achieving a picture of high resolution without increasing the number of pixels.
However, with such solid-state image pickup devices, since the image pickup light radiated on the CCD image sensor is image-shifted by mechanical vibrations applied to the CD image sensor itself or to the glass plate mounted ahead of the CCD image sensor, not only the mechanical structure becomes complexicated but also the image shifting is only low in reliability.
For obviating the defect inherent in the solid-state image pickup device effectuating the image shifting by mechanical means, there has also been developed a solid-state image pickup device in which image shifting may be realized by electrical means.
The solid-state image pickup device with electrical image shifting is arranged as shown for example in FIG. 1. When the image pickup is started with the solid-state image pickup device, the image pickup light is converged by a lens 50 so as to be radiated on a birefringence plate 51.
When irradiated with the image pickup light from lens 50, the birefringence plate 51 splits the light into an ordinary light and an extraordinary light having an optical path shifted from the optical path of the ordinary light vertically upwards by a pitch equal to one-half pixel so as to cause these light rays to be radiated on a voltage-control led optical path selecting plate 52.
The voltage-controlled optical path selecting plate 52 is made up e.g. of a liquid crystal plate and a polarization plate. An electrical voltage is applied to the liquid crystal plate at a one-field interval. When the electrical voltage is not applied to the liquid crystal plate, the ordinary light and the extraordinary light, radiated to the liquid crystal plate via the birefringence plate 51, are directly transmitted through the liquid crystal plate and only the ordinary light is transmitted through the polarization plate. When the electrical voltage is applied to the liquid crystal plate, the ordinary light and the extraordinary light, radiated to the liquid crystal plate via the birefringence plate 51, are polarized by 90.degree. when passed through the liquid crystal plate, with the respective optical paths remaining unchanged. Of these light rays, solely the extraordinary light is transmitted through the polarization plate. In this manner, the ordinary light or the extraordinary light is selectively radiated from the voltage-controlled optical path selecting plate 52, at a one-field interval, so as to be radiated on a CCD image sensor 53.
The CCD image sensor 53 receives the ordinary light and extraordinary light to effect photoelectric transfer to generate image-pickup signals which are outputted at an output terminal 54. It is noted that not only the ordinary light but also the extraordinary light having an optical path shifted vertically upwards by a pitch equal to one-half pixel with respect to the optical path of the ordinary light is radiated on the CCD image sensor 53.
This leads to an increased spatial sampling area and to increased resolution.
As compared to a solid-state image pickup device, effectuating the image shifting by mechanical means, the solid-state image pickup device, effectuating the electrical image shifting, is devoid of moving parts of a complicated mechanical system, and hence leads to improved reliability in the image shifting and to a simplified structure of the device in its entirety.
However, with the above-described solid-state image pickup device, the image pickup light is split by the birefringence plate 51 into the ordinary light and the extraordinary light and the electrical voltage applied across the selecting plate 52 is on/off controlled for selecting one of the ordinary light and the extraordinary light from the birefringence plate 51 for radiating the selected light to the CCD image sensor 53. Consequently, the image-pickup light radiated on the CCD image sensor 53 is reduced to substantially one-half the original image pickup light so that only a dark image may be produced.
On the other hand, the recent tendency is towards an increasingly small size of a video camera unit etc. and a demand has been raised for the electrically image-shifting type solid-state image pickup device fabricated in one chip.